Micro- or nanostructured optical frequency filters are generally preferred because of their great compactness which enables them to be integrated in photodetectors. Such filters usually comprise a support layer having a metallic grating of slits smaller than the wavelength to be filtered formed thereon. The amplitude of the transmission is determined by both the width and the thickness of the slits, the thickness being selected to be at least equal to half the considered wavelength. Such a thickness for example corresponds to several hundreds of nanometers in infrared.
Further, in this type of detector, the refractive index of the support is systematically selected to be as small as possible in order to disturb the slit grating transmission as little as possible. The search for the lowest possible refractive index has thus resulted in filters with a metallic slit grating directly suspended above air (refractive index close to 1), which is difficult to achieve technologically.
It is also remarkable that there is no further degree of liberty to adjust the shape of the transmission profile (amplitude and width of the transmission band, or transmission “peak”), the characteristics of the grating (periodicity, slit thickness and width) being essentially used to position the transmission band in the spectrum.
Thereby, the amplitude and the rejection factor of this type of filters are imposed and are typically quite unsatisfactory. In particular, the rejection factor is smaller than 90%, and in most cases smaller than 80%, outside of the bandwidth of interest.
This type of filter has for example being described by H. Lochbilher and R. Depine, “Highly conducting wire gratings in the resonance region”, Applied Optics, 32, p. 3460, 1993, by J. A. Porto, F. J. Garcia-Vidal and J. B. Pendry, “Transmission resonances on metallic gratings with very narrow slit”, Physical Review Letters, 83, p. 2845, 1999, by G. Vincent et al., “Large-area dielectric and metallic freestanding gratings for midinfrared optical filtering application”, Journal of Vacuum Science Technology, B 26, p. 852, 2008, by A. Barbara et al., “Optical transmission through subwavelength metallic gratings”, Physical Review, B 66, p. 852, 2002, or again in documents U.S. Pat. No. 7,420,156, US 2009/0073434 and WO 2007/118895.
T. Zentgraf et al.'s publication, “Ultranarrow coupling-induced transparency bands in hybrid plasmonic systems”, Physical Review, B 80, p 195415, 2009 describes a periodic grating of thin strips, formed on a transparent substrate at the considered wavelength, and embedded in a dielectric layer forming a waveguide. The total strip surface area does not exceed 20% of the total surface area of the grating and the transmission response is set by the grating dimensions as well as by the height of the dielectric layer where the grating is embedded.
The obtained transmission peak is very thin, with a width on the order of 1% of the wavelength to which the filter is tuned at mid-height of the peak, of high amplitude, close to 90% at this wavelength, and with a rejection factor around the transmission peak close to 90%.
However, the gain in terms of liberty obtained by introducing the waveguide combined with a thin strip grating is essentially obtained for the shape of the transmission profile. Thus, the spectral range where a good rejection is provided around the transmission peak is very limited, between 845 and 855 nanometers, while many applications require a spectral rejection range having a width on the order of half a wavelength, that is, 400 nm or more in the present example. Further, the rejection factor around the peak still remains too low for certain applications.
Moreover, this filter only transmits the electromagnetic radiation component polarized parallel to the grating strips, and it is not possible to transmit the orthogonally-polarized component. It has indeed been shown that the stacking of thin orthogonal strips, respectively forming a first grating for the first polarization and a second grating for the second orthogonal polarization, is not operative. Similarly, a metallic grating formed of rectangular pads, defining two orthogonal gratings, is not operative either. The only solution to transmit two different polarizations then is to juxtapose two gratings of strips of different orientation, which adversely affects the filter compactness.
Finally, the filter is tuned according to the thickness of the dielectric layer where the grating is embedded.
The forming of a multiple-frequency filter having several transmission bandwidths thus requires forming a dielectric layer of variable thickness. Such a construction increases the number of manufacturing steps and adversely affects the subsequent integration of the filter in a circuit such as, for example, a photodetector.